Steel pipe of the type used, for example, in oilfields, is sometimes subject to severe mechanical stress and stress corrosion. Proper quenching of the pipe after it has been formed tends to increase the resistance of the pipe to failure. It is known to quench this type of pipe by immersing it in a bath of liquid cooling medium, such as water, and causing the liquid cooling medium to flow both through the interior of the pipe and around the exterior of the pipe in controlled proportions, hereinafter referred to as "inside-outside" quenching.
This inside-outside quenching technique, and apparatus for practising same, are the subject of U.S. Pat. Nos. 3,997,375 and 3,877,685, and corresponding Canadian Pat. No. 1,016,148, granted to the assignee herein. Briefly summarized, the method comprises supporting the hot pipe horizontally in an elongated tank and passing part of the liquid cooling medium flow (typically water) from an inlet nozzle directly through the inside of the pipe and passing another part of the cooling medium flow directly from the inlet over the outside of the pipe. The relative proportions of the flow of cooling medium from the inlet directly through the pipe and of the flow over the outside of the pipe are varied to achieve a desired hardening effect. Typically, the desired effect is substantial uniformity of hardness as between the inside and outside surfaces of the pipe, as is required by some oil industry specifications.
Suitable apparatus of this known type may comprise an elongated tank dimensioned to receive the hot pipe section to be quench-hardened, and fins or other suitable supporting elements for supporting the hot pipe section horizontally in the tank and spaced from the bottom of the tank to permit adequate water flow around the pipe section. A nozzle having a tip, for introducing water into the pipe section is connected to a water supply. The nozzle is mounted for moving the nozzle between a retracted position in which the tip is spaced from one end of the pipe (to allow insertion and removal of the pipe section into and from the container without interfering with the nozzle), and an extended position in which the tip lies within the end of the pipe section (so as to direct water flow in a concentrated stream into the interior of the pipe section). The inlet and nozzle arrangement is configured to direct a flow of water also around the outside of the pipe section. Since varying pipe diameters and variation in other operating parameters may require some variation in relative interior/exterior water flow in order to maintain uniform inside/outside hardness of the quenched pipe section, suitable valving arrangements are provided to vary the proportion of water entering through the nozzle into the interior of the pipe section relative to the proportion of water passing over the outside of the pipe section, so as to control the rate of cooling of the inside surface relative to the outside surface of the pipe section. Conventional lifting arms or other means for removing the pipe from the elongated tank after quenching are also provided. This apparatus constitutes an improvement of earlier known apparatus described in U.S. Pat. No. 3,623,716 granted to Mannesmann Tube Company Ltd.
The inside-outside quenching method and apparatus are described in detail in the above-mentioned patents, the disclosures of which are hereby incorporated herein by reference.
One of the visible problems associated with conventional immersion quenching of pipe is "warping", or lack of straightness, in the quenched pipe. In immersion quenching, a container filled with a cooling medium receives a hot pipe section dropped or lowered into the medium before coolant is injected into the interior of the pipe.
In order to harden low alloy steels effectively, each region of the metal, whether interior or close to the surface, must be cooled from its austenitizing temperature (typically 1600.degree. F.) to its Ms temperature (typically 500.degree. F.) within a matter of seconds and in an uninterrupted manner once quenching commences. One of the inherent deficiencies of immersion quenching is that it entails a transient immersion quench period which makes up a substantial part of the duration of contact between the pipe and the cooling medium. This transient immersion quench period (or "slack quench") occurs from the time of the first contact of the pipe with the surface of the cooling medium to the time when full steady state quench flow is established.
From the moment the pipe first contacts the cooling medium in a substantially horizontal position, the cooling medium flows freely but relatively slowly into the inside of the pipe while air is escaping therefrom. The cooling of the inside pipe surface occurs by conductive heat transfer through a vapour blanket and is of the order of 1/10 the cooling rate at full turbulent flow, and is non-uniform as some portions of the inside pipe are first contacted by cooling medium substantially in advance of other portions.
The initial slow cooling described above occurs during the period in which the pipe moves to the bottom support of the tank, is secured in position, a nozzle for the introduction of coolant into the interior of the pipe is advanced into the end of the pipe, and a valve is actuated to release coolant through the nozzle, before full turbulent flow can flush out the pool of cooling medium which has entered the inside of the pipe from one or both ends. Portions of the inside of the pipe in contact with this pool will not be fully hardened and, owing to premature transformation, will tend to cause distortion of the pipe.
It has been found by experimentation that a further characteristic of conventional immersion quenching is that outside quench effectiveness does not match inside quench effectiveness, with differential hardening of the pipe as a result. The differential of volumetric expansion between the exterior portions and the interior portions of the pipe occurring during quenching not only tends to create distortion, but also creates severe internal stresses giving rise to the possibility of cracking of the finished pipe in use with certain chemical compositions. The mismatch between inside and outside quench effectiveness that arises in conventional immersion quenching is especially pronounced in the case of small-diameter, small bore tubing. External quenching of the pipe is conventionally effected simply by the free flow of cooling medium in the open tank over the outer surface of the tube. As progressively smaller sizes of pipe are quenched, the outside cooling efficiency of the available volume of cooling medium free flowing in the open tank decreases, making quenching of small diameter, small bore tubing quite impractical since the effectiveness of the inside quench (effected by means of a suitable nozzle injecting cooling medium into the pipe at a substantial pressure) cannot be properly matched to the outside cooling efficiency.
As is well known, the quenching process is attended by volumetric changes in the steel, for example the expansion attendant on the martensitic transformation, and it is believed that the warping problem encountered in practice, results from non-uniformity in the cooling of the workpiece, leading to non-uniform volumetric change rates. For given quenching apparatus, it has been found that the non-straightness problem appears more serious for pipe diameters at the low and high ends of the range of diameters for which the apparatus is used, and less serious for intermediate diameters. It is suspected that part of the problem with small diameters results from the restricted interior dimension impeding the flow of the quenching medium inside the pipe, resulting in a less satisfactory inside quench; the problem with larger diameters results largely from coolant partially filling the inside of the pipe before a full steady state inside flow is established and partly from the volume of flow of coolant relative to the inner and outer surface areas of the pipe being smaller than is the case when intermediate diameters are quenched using the same apparatus.
Previously disclosed apparatus and methods for immersion quenching purport to eliminate problems with the non-straightness of pipe by a number of expedients. One such approach aims to eliminate the non-straightness problem by commencing the internal quench very shortly after the pipe is rigidly clamped at spaced positions along its entire length to restrain it from warping. This approach is exemplified in the teachings of U.S. Pat. No. 4,116,716 (Itoh). There is a risk that simply restraining the pipe from warping during a non-uniform quench may have the undesired effect of preventing the natural relief of internal stresses occurring during warping and, as a result, inducing active stresses during the quenching process and residual stresses therafter.
It has also been proposed to eliminate problems with the non-straightness of pipe in an immersion quenching technique by commencing the internal and external quench very shortly after the pipe has been introduced into a coolant-filled tank containing a housing for holding the pipe in position during the quenching process. The Ohshimatani U.S. Pat. No. 4,376,528 teaches that it is preferable that the housing for inside-outside quenching be itself submerged in cooling water in the quench tank before the hot pipe is introduced into the housing so that the cooling water functions as a damping medium to reduce the speed of approach of the dropping steel pipe to the housing. It is further suggested by Ohshimatani that dropping the hot pipe into water for a period of time before injecting coolant eliminates soft spots in the hardened steel because cooling water instantaneously enters the steel pipe. Applicants' experience with inside-outside quenching of pipe indicates that the above teaching of Ohshimatani points in entirely the wrong direction. The "slack quenching" that occurs even during a very brief period of submersion of a hot pipe in stationary coolant is unsatisfactory for the reasons discussed above.